The reaction between ammonia and methyl bromide, which has been taken as a model for the Menshutkin reaction, has been studied through ab initio methods in the gas phase and in solution using discrete and continuum representations of the solvent. The solvent effect on this reaction bears a resemblance with other SN2 reactions but also exhibits some differences with them. The main results turn out to be, on one hand, a decrease in the energy barrier upon increase in solvent polarity, and, on the other hand, the transition state is found earlier along the reaction coordinate, showing the participation of solvent parameters in the reaction coordinate. The polarization of the solute by the reaction field created by the solvent polarization is one of the most important aspects of the coupling between the solvent and the chemical system. This translates into an increase of the weight of the charge-transfer configuration with respect to the weight in the gas phase. Likewise, fluctuations increasing the reaction field are stabilized by instantaneous changes in the electronic distribution of the solute. © 1991, American Chemical Society. All rights reserved.